In the electrolysis of aqueous alkali metal halide solutions in electrolytic cells having a diaphragm or membrane separator, the applied voltage required is the total of the decomposition voltage of the compounds being electrolyzed, the voltage required to overcome the resistance of both the electrolyte and the electrical connectors of the cell, and the overpotential required to overcome the passage of current at the surface of the cathode and the anode. The overpotential is related to such factors as the nature of the ions being charged or discharged, the current density at the electrode surface, the base material from which the electrode is constructed, the surface formation of the electrode, i.e., whether the electrode is smooth or rough, the temperature of the electrolyte, and the presence of impurities in the electrolyte and the electrodes. At the present time, knowledge of the phenomenon of overpotential is not fully understood. It has been observed that there is a characteristic overpotential for each particular combination of discharging ion, electrode, electrolyte, current density, etc.
Because of the large quantities of chlorine and caustic (sodium hydroxide) required by a modern society, millions of tons of these materials are produced, principally by electrolysis of aqueous solutions of sodium chloride, each year. A reduction of as little as 0.05 volts (50 millivolts) in the working voltage of a cell translates into a meaningful economic savings, especially in the light of today's increasing power costs and energy conservation measures. As a result, the electrochemical industry is constantly in search of means which will reduce the voltage requirements for such electrolytic processes.
The development of the dimensionally stable anode and coatings therefor have resulted in a reduction in the anode and cathode spacing within electrolysis cells, this advance resulting in a large reduction in the voltage since electrolyte resistance is reduced within the narrow space between the electodes.
Cathodes for electrolysis cells are generally made of a mild steel generally in the form of expanded mesh, screen, or perforated plate because of the low cost of this material and its resistance to the caustic environment of the catholyte.
Various coatings have been proposed for reducing the hydrogen discharge overpotential at the electrode surface of electrolysis cells.
U.S. Pat. No. 3,632,498, Beer, describes a coating comprising a solid solution of precious metal oxides and film-forming metal oxides on a film-forming metal substrate to be used as an anode in electrolytic processes. Similarly, U.S. Pat. No. 3,711,385, Beer, describes a mixed crystal anode coating on a film-forming metal base comprising oxides of platinum metals group with oxides of film-forming metals.
Bennett, et al, U.S. Pat. No. 3,677,975, also describes a coating comprising a solid solution of a valve metal dioxide and a precious metal dioxide applied to a valve metal substrate. A coated electrode in accordance with this invention may be used as an anode in electrolytic processes.
Moss, U.S. Pat. No. 3,869,312, describes a coating for an anode comprising a film-forming metal substrate, a first layer comprising a mixture of a platinum group metal and a film-forming metal oxide and a second layer of coating consisting of a film-forming metal oxide.
In all of the above-mentioned patents, the preferred mixed oxide coating is applied only to a film-forming metal substrate and is generally used only as an anode coating for electrolytic processes since hydrogen embrittlement of valve metal substrates occurs when such coated electrodes are used as cathodes. Further, the coating mixtures are applied as a solution utilizing an organic solvent such as alcohols, e.g. isopropanol or n-pentanol. The utilization of such organic solvents not only increases the cost of the coatings but also presents a health and fire hazard.
As used in this specification, film-forming metals will be understood to include metals which form a protective film on their surfaces such as aluminum and the valve metals titanium, tantalum, zirconium, niobium and vanadium. Precious metals include principally gold, silver and the platinum group metals platinum, palladium, rhenium, ruthenium, osmium and iridium.
U.S. Pat. No. 3,654,188, Kolb, describes a process for preparing solid solutions of valve metal dioxides and precious metal dioxides independent of a valve metal substrate. The patent states that prior to its disclosure, it was necessary that such solid solutions be formed on a valve metal substrate and that attempts to form such a solid solution on other substrates resulted in only loosely adherent physical mixtures of the oxides in seperate crystaline phases.
It has been found that the hydrogen discharge overpotential at the cathode surface is lowered by a coating comprising a mixture of precious metal oxides and valve metal oxides, however, the coating has formerly not found commercial acceptance because of the aforementioned hydrogen embrittlement of the valve metal substrates upon which these coatings were necessarily formed.